Different handing of "this" vs. "MyClass.this" in definite assignment

[Brian Goetz]

I can think of multiple explanations for why this wasn't done in 1.1 
when the `MyClass.this` syntax was introduced, but I think there's a 
little more detangling to be done before we have a clear menu of 
options, because we have to consider that `Example1.this` only means 
`this` when there are no intervening class declarations between the use 
and the declaration of Example1.

To come up with a sensible meaning of option (2), I think we have to 
assess the spec impact of threading the equivalence of `[implicit 
C].this` and `C.this` through the spec?



On 12/1/2022 1:09 PM, Archie Cobbs wrote:
> This relates to JDK-8193904 
> <https://bugs.openjdk.org/browse/JDK-8193904> (if amber-dev is the 
> wrong place to ask about this please let me know; here's an earlier 
> thread 
> <https://mail.openjdk.org/pipermail/compiler-dev/2022-November/020899.html> 
> on compiler-dev).
>
> The bug asks the question, should we change the JLS so that 
> "MyClass.this" is treated the same way as "this" with respect to 
> definite assignment?
>
> Here's how the compiler currently behaves:
>
> // This class compiles without error
> public class Example1 {
>     final int foo;
>     public Example1() {
> System.err.println(Example1.this.foo);  // no error generated here
>         this.foo = 42;
>     }
> }
>
> // This class fails to compile
> public class Example2 {
>     private final int foo;
>     public Example2() {
> Example2.this.foo = 42; // "cannot assign a value to final variablefoo"
>     }
> }
>
> Presumably the reason for the different treatment is because in 
> general the qualifying expression could be arbitrarily complex and the 
> compiler can't be expected to detect any possible qualification in 
> front of "foo", so why even start down that road?
>
> For example, instead of
>
> Example1.this = 42;
>
> you could have:
>
> ((Example1)Function.<Object>identity().apply(this)).foo = 42;
>
> Currently JLS §16 says:
>
>     An access to its value consists of the simple name of the variable
>     (or, for a field, the simple name of the field qualified by
>     |this|) occurring anywhere in an expression except as the
>     left-hand operand of the simple assignment operator |=| (§15.26.1
>     <https://docs.oracle.com/javase/specs/jls/se19/html/jls-15.html#jls-15.26.1>).
>
>
> Some options...
>
>  1. Deprecate the use of "MyClass.this" where "this" would work and
>     start generating a warning
>  2. Update JLS §16 to say "qualified by this or qualified by MyClass.this"
>  3. Do nothing and resolve JDK-8193904 as "Won't Fix"
>  4. ??
>
> Thoughts?
>
> -Archie
>
> -- 
> Archie L. Cobbs
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